These in vivo studies are designed to 1) characterize the estrogen receptor system in the hypothalamus, pituitary and uterus of the rat as a function of age and reproductive state. Qualitative properties of the different forms of estrogen receptor, their compartmentalization, and the process of intranuclear transformation will be assessed by sucrose density gradient ultracentrifugation. Quantitation of the total tissue estrogen receptor content will be effected by use of the cytoplasmic and nuclear exchange assays. The physiologic process of developing pituitary sensitivity to a hypothalamic influence as mediated by estrogen milieus will be studied in both the pre and postpubertal state. The relationship between the total pituitary estrogen receptor content, its qualitative aspects, and the responsivity of the gland to LRF (measured by serum LH content) will be determined.